In nuclear reactors which have power and shutdown control rods, it is known to use tubes not only to guide the rods themselves which are in the form of clusters, but also to guide the shaft connecting the cluster to the mechanisms controlling the translational movement of the rods. This shaft, called a follower, is centered and guided in a tube arranged in the space contained between the tubes guiding the clusters and the wall of the containment carrying the mechanisms moving the control rods.
Arranged in a known way in the end part of the tube is a ring which ensures, on the one hand, the guidance of the follower and, on the other hand, the limitation of the flow of coolant which has passed through the core of the nuclear reactor. The stream of coolant is thus made to flow off via the orifices provided in the cluster guide tubes for it to pass through.
Furthermore, during the operations of unloading and refuelling the reactor core, the ring associated with the guide tube makes it possible to guide the corresponding follower, in such a way that, when the cover of the vessel is lowered, this follower can enter the corresponding passage provided in the cover.
Finally, this ring must be removable, in order to make it possible to carry out the conventional maintenance operations on the components of the core of the nuclear reactor, and must be lockable relative to the guide tubes, so that it is maintained in a fixed position during the normal operation of the reactor.
There are known embodiments, in which the ring is held in an orifice passing through the end plate of the guide tube, in such a way as to be coaxial relative to this orifice, by maneuverable attachment means retained in place by an elastic means. The guide ring comprises a tubular body, whose first part enters the orifice in the end plate of the guide tube and whose second part comes up against the upper face of this plate. The attachment means mounted on the tubular body engage with the edges of the orifice in the plate on the lower face of the latter.
The tubular body of the guide ring consists of two coaxial rings, of which one, the inner ring, is mounted slideably in the axial direction inside the other which forms the outer ring. The internal bore in the inner ring forms the guide surface for the follower. The outer ring carries the locking means which usually consist of attachment fingers mounted pivotably about horizontal axes, and the inner ring has, on its outer surface, surfaces actuating the pivoting fingers, these surfaces being designed to put these fingers into the locked or released position as a result of the axial displacement of the inner ring. An elastic means, such as a spring, is inserted between the outer ring and the inner ring, so as to exert a restoring force on the inner ring, to maintain it in the position corresponding to the locking of the fingers.
The disadvantage of such guide rings is that they consist of two parts movable relative to one another and require the use of a special tool to exert a thrust on the inner ring, to ensure the release of the fingers at the moment when the guide ring is fitted on the end plate of a guide tube.